A razor or utility knife is known whose blade retracts automatically if the knife is subjected to an acceleration above a certain threshold. Thus if the knife slips, so that it is moved very rapidly, the blade retracts into the grip body or handle. Since such acceleration normally only happens when the knife becomes disengaged from the object it is cutting and can therefore lead to an accident, this system protects the user.
In the system of EP 244,517 of R. Davis the retraction is effected by a tension spring and the blade is retained in its extended position by an inertial mass which frees the blade when subjected to excessive acceleration to allow the spring to retract it. The latching mechanism is formed by a detent tooth on an extension of the blade and a complimentarily formed seat in a lever on the grip body formed as the inertial mass that is loaded by a second spring as the blade is extended from the grip body to engage the detent tooth in its seat. If the knife with the handle is subjected to an unacceptable accelerated movement and the inertial forces of the one arm lever exceed the locking forces of the second spring of the latching mechanism as well as the outwardly effective friction, it is unlatched and the blade is pulled by the cylindrical coil spring back into the grip body.
U.S. Pat. No. 5,203,085 describes a system which has a latching spring of the latching mechanism formed as a longitudinally extending tension spring. The flat latching surface only actuated by the tip of the detent tooth extends when the blade is extended with its longitudinal axis generally tangentially to a circle centered on the pivot of the latching mechanism. The return forces determined by the sizes of the longitudinally stretched weak tension spring change very little in a mass-produced knife so that the result is an always reproducible response sensitivity of the latching mechanism. In addition this knife is known for the high threshold which is responsible for the cutting work and which prevents a retraction under load of the blade which is already slowed by the usual cutting work. This response threshold is determined by the tension of the latching spring in spite of the above-described tangential movement on the flat latching surface which appears to permit only a temporary stopping of the latching device through the known drawing-in of the limiting friction along with the latching device and the holding projection.
In these known knives the response threshold is always established purely mechanically so that it is necessary to take into account the sum of all friction forces that affect the inertial mass of the latching device, the latching spring, the bearings as well as their sliding and rolling friction. In case of accident the return speed into the grip body depends not only from the size of the energy store, which here is the loading of the coil spring effective on the extension, but also of the frictional and inertial forces of the purely mechanically acting latching device and its unlatching elements.
Furthermore, it has been shown that in particular applications the response sensitivity of the latching mechanism can vary over a wide range. Furthermore, the system is normally only responsive to acceleration in one direction, normally crosswise of the elongated body of the knife.